Scanning apparatus

ABSTRACT

Scanning apparatus which scans input radiation from a scene and output radiation is transmitted to a receiver system, for example a millimetre wave imaging camera or a radar receiver by a rotatable reflective plate having an axis of rotation passing through the centre of its surface, secondary reflector and static reflector, wherein the secondary reflector is a second rotatable reflective plate having a common axis of rotation with the first rotatable reflective plate, wherein the common axis of rotation is inclined at a non-zero zero angle θ b  to the normal to the second reflective plate. The normal to the first rotatable plate is inclined at a small angle to the common axis of rotation, typically a few degrees and forms the secondary reflector. The static reflector may be a polarising roof reflector through which radiation is input to and output from the apparatus. The apparatus also includes a  45 ° Faraday rotator or a birefringent surface such as a Meander-line. An additional Faraday rotator and an inclined polariser may be included in the apparatus and arranged such that radiation output to the receiver system may be separated from the path of input radiation. Alternatively, the scanning apparatus may include a reflector lens arrangement, such that focused output radiation may be output directly to the receiver system.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to scanning apparatus which may be used in areal-time imaging system and, in particular, in a real-time passivemillimetre wave imaging system. The scanning apparatus may also be usedin other radiometry systems.

2. Discussion of Prior Art

British Patent No. 700868 (February 1952-December 1953) describes atwistreflector which relates to a similar field as the presentinvention.

Millimetre wave imaging is potentially useful as an all-weathersurveillance and guidance aid but any practically useful system must becapable of imaging in real-time. This is not possible using existingsystems. In a millimetre wave imager, radiation from the scene to bescanned is collected by means of a concave mirror or a lens and isfocused onto an array of millimetre wave receivers. At present, largetwo-dimensional arrays of receivers which cover the whole of a requiredimage are not available. Instead, a far smaller number of receivers isscanned across the image in order to build up the complete picture. Asimilar technique is used in some infrared imagers (for example EP0226273).

Current millimetre wave imaging systems use mechanical scanning of oneor several channels to synthesise an image. Ultimately, electronicscanning and staring array techniques could be developed to implementreal-time millimetre wave imaging, although there are several problemsassociated with such a solution. Firstly, as the wavelength isnecessarily long, in order to image under adverse weather conditions thesystem aperture must be large to gain adequate resolution. In somemillimetre wave imaging systems the input aperture may be of the orderof 1 m in diameter. Secondly, the cost per channel is high so that anyelectronically scanned or staring array technique is expensive.Furthermore, in the case of millimetre wave staring arrays there arefundamental problems analogous to the cold shielding problemsencountered in infrared systems.

Another requirement of a practical millimetre wave imaging system isthat it must be able to operate at TV-compatible rates (i.e. 50 Hz forthe UK, 60 Hz for the USA). In the infrared, scanning systems are oftenplane mirrors flapping about an axis contained within their surface.This is not a practical option in the millimetre waveband as largeaperture mirrors would be required to flap back and forth atTV-compatible rates, requiring a large change in inertia at the end ofeach scan.

In infrared imaging systems, where input apertures are typically only 10mm in diameter, rotary systems have been used (EP 0226273). Furthermore,in the infrared, it is usual to employ afocal telescopes to match thefield of view in the scene to that of the rotating polygon. This isimpractical in high resolution millimetre wave imaging where the inputapertures have considerably greater diameters and afocal telescopeswould need to be excessively large.

Any scanning mechanism used in a millimetre wave imaging system musttherefore be situated in either the object or the image plane.Furthermore, any scanning mechanism situated in the image plane musthave good off-axis performance. This is difficult to achieve usingexisting technology.

Another known scanning method used in infrared imagers is a system oftwo discs rotating about axes which are slightly inclined to the normalsto their faces. Radiation incident on the first disc is reflected atoblique incidence from the first rotating disc and passes to the seconddisc to experience a second reflection. By varying the orientation andrelative speed of rotation of the discs, varying scan patterns can beachieved. Such a two-axis rotating disc system would not be ideal foruse in millimetre wave imaging, however, as the system would beinconveniently large.

It is an object of the present invention to provide a compact objectspace scanning apparatus which may be used, in particular, to implementreal-time millimetre wave imaging, or in radar systems. It is also anobject of the invention to provide a scanning apparatus which haslimited power requirements and minimum inertia and gives good off axisperformance.

According to the present invention, apparatus for scanning radiationfrom a scene and for generating output radiation for input to a receiversystem comprises;

a first rotatable reflective plate, for receiving and reflectingradiation, having an axis of rotation passing substantially through thecentre of the plate, wherein the axis of rotation is inclined at anon-zero angle θ_(a) to the normal to the reflective plate,

rotary means for rotating the reflective plate,

secondary reflection means for receiving and reflecting radiation and

static reflection means for receiving radiation reflected from the firstrotatable reflective plate and reflecting radiation towards thesecondary reflection means,

characterised in that the secondary reflection means is a secondrotatable reflective plate having a common axis of rotation with thefirst rotatable reflective plate, wherein the common axis of rotation isinclined at a non-zero angle θ_(b) to the normal to the secondreflective plate.

The scanning apparatus provides the advantage of compactness. It hasminimum inertia and minimum power requirements. The apparatus may besituated at the entrance pupil of an imaging camera or receiver andprovides good off-axis performance.

SUMMARY OF THE INVENTION

The apparatus may also include a millimetre wavelength imaging camera ora radar receiver.

The normals to the first and second reflective plates are inclined insubstantially the same plane and at substantially equal angles to thecommon axis of rotation and in substantially opposite directions.Typically, the angles of inclination θ_(a), θ_(b) may be between 1° and10°.

The static reflection means may comprise a plane mirror having areflective surface substantially parallel to the common axis ofrotation.

In another embodiment of the invention, the secondary reflection meansmay be the first rotatable reflective plate. Typically, the axis ofrotation may be inclined at an angle of between 1° and 10° to the normalto the reflective plate.

In this embodiment of the invention, the static reflection means maycomprise two reflective surfaces inclined at substantially 90° to eachother. Preferably, the two reflective surfaces form a roof reflector,such that the two reflective surfaces are in contact along an apex.

The apparatus may also comprise a polarising mirror arranged to reflectoutput radiation to the receiver system. The polarising mirror may be asheet of plastic material comprising a plurality of parallel conductingwires, wherein the parallel conducting wires are oriented atsubstantially 45° to the apex of the roof reflector.

Alternatively, the static reflection means may comprise two polarisers,each having a polarisation axis, inclined at substantially 90° to eachother. Preferably, the two polarisers form a polarising roof reflectorsuch that the two polarisers are in contact along an apex and thepolarisation axes of the polarisers are oriented to transmit radiationhaving substantially the same direction of polarisation, wherein saiddirection of polarisation is substantially parallel or substantiallyperpendicular to the apex.

In an alternative arrangement, the static reflection means may comprisea plurality of polarising roof reflectors, each comprising twopolarisers and each polariser having a polarisation axis, wherein saidpolarisers are inclined at substantially 90° to each other and are incontact along an apex,

wherein the polarisation axes of the polarisers forming each roofreflector are oriented to transmit radiation having substantially thesame direction of polarisation wherein said direction of polarisation issubstantially parallel or substantially perpendicular to the apexes.

The apparatus may also comprise a first Faraday rotator, situatedbetween the polarising roof reflector and the rotatable disc, forrotating the direction of polarisation of radiation throughsubstantially 45° each time the radiation passes through the Faradayrotator,

such that the radiation having a particular direction of polarisationmay be output through the polarising roof reflector.

Alternatively, the apparatus may comprise one or more birefringentsurfaces, such as a Meanderline, situated between the polarising roofreflector and the rotatable disc, for receiving radiation in a state ofpolarisation, P_(s,)

whereby the one or more birefringent surfaces introduce a substantially90° phase shift in the state of polarisation, P_(s), each time radiationpasses through the one or more birefringent surfaces, such thatradiation having a particular direction of polarisation may be outputthrough the one or more polarising roof reflectors.

The apparatus may also comprise means for selectively transmittingradiation input to the apparatus having a particular direction ofpolarisation and for selectively reflecting radiation output from theapparatus having a particular direction of polarisation to the receiversystem.

For example, the apparatus may comprise;

a second Faraday rotator for rotating the direction of polarisation ofradiation output from the polarising roof reflector throughsubstantially 45°

and may also comprise a second polariser, wherein the second polariserhas an axis of polarisation inclined at substantially 45° to the apex ofthe polarising roof reflector.

Alternatively, the apparatus may comprise a lens arrangement forselectively transmitting and focusing radiation having a particulardirection of polarisation. The lens arrangement may be a reflector lenscomprising a first polarising surface having a polarisation axis, forselectively transmitting and selectively reflecting radiation having aparticular direction of polarisation, a second surface for rotating thedirection of polarisation of radiation through substantially 45° and athird polarising surface for selectively reflecting and selectivelytransmitting radiation, wherein the third polarisation axis makes anangle of substantially 45° with the first polarisation axis. Thisprovides the advantage that radiation output from the apparatus isfocussed and therefore may be output directly to a receiver system. Theapparatus may comprise two or more such lens arrangements arranged inseries.

In this embodiment of the invention, the static reflection means mayform part of the lens arrangement, for selectively transmitting andfocussing radiation having a particular direction of polarisation, theapparatus being arranged to provide a conical scanning apparatus.

In a particular arrangement of this embodiment of the invention thefirst polarising surface may have a substantially flat surface and thethird polarising surface may have a substantially spherical surfacehaving a radius of curvature, R, and the apparatus may also comprise adetector array forming part of a spherical surface having half theradius of curvature of the spherical surface of the third polarisingsurface and being concentric with it.

The apparatus may also comprise a corrector plate located between therotatable disc and the third polarising surface for removing aberrationsarising from an image formed at the detector array.

According to a second aspect of the invention, a reflector lens maycomprise;

a first polarising surface having a polarisation axis, for selectivelytransmitting and selectively reflecting radiation having a particulardirection of polarisation,

a second surface for rotating the direction of polarisation of radiationthrough substantially 45° and

a third polarising surface for selectively reflecting and selectivelytransmitting radiation,

wherein the third polarisation axis makes an angle of substantially 45°with the first polarisation axis.

At least one of the first, second or third surfaces may have a curvedsurface.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described, by example only, with reference tothe following figures in which;

FIG. 1 shows a diagram of a conventional rotating two axis, two discsystem,

FIGS. 2(a) and 2(b) show examples of the scan patterns which may beachieved using the rotating two disc system in FIG. 1,

FIG. 3 shows the single axis two disc system of the present invention,

FIG. 4 shows a one disc scanning system comprising a roof reflector,

FIG. 5 shows a roof reflector,

FIGS. 6 shows a compact one disc scanning systems comprising apolarising roof reflector,

FIG. 7 shows a schematic diagram of a reflector lens which may be usedin the scanning apparatus,

FIG. 8 shows a diagram of a one disc scanning system, including thepolarisation sensitive reflector lens in FIG. 7,

FIG. 9 shows a one disc scanning system employing a plurality of roofreflector elements,

FIG. 10 shows a near linear open scan pattern,

FIG. 11 shows an embodiment of the apparatus which may be used toprovide a conical scan pattern and

FIG. 12 shows the scan pattern which may be achieved using the apparatusshown in FIG. 11.

DETAILED DISCUSSION OF PREFERRED EMBODIMENTS

Referring to FIG. 1, a conventional two disc rotating system comprisestwo discs 1 a, 1 b, each supported on a separate axis 2 a,2 b which isconnected to a rotor mechanism 3 a,3 b. Each axis 2 a,2 b is inclined afew degrees to the normals to the faces of the discs 1 a,1 b. Typicallythe angle of inclination is 5°. As the discs 1 a, 1 b rotate about theirrespective axes, incident radiation 4 from the scene is incident on thefirst rotating disc 1 a and is reflected at oblique incidence towardsthe second rotating disc 1 b where it experiences a second reflection.From the second rotating disc 1 b, radiation may be passed to an imagingor receiving system, typically comprising collection optics 5 and areceiver 6 (or receiver array). For example, the receiver 6 may be thereceiver element of a millimetre wave imaging camera or the receiverelement of a radar system.

The two discs 1 a,1 b may be inclined at the same or different angles tothe normal to the respective disc face and may rotate with the same ordifferent speeds, depending on the scan pattern required at the imager.If the two discs 1 a, 1 b are inclined at different angles to their axesof rotation and are rotated at different speeds, a two-dimensional scanpattern will be achieved. If the angles of inclination of the two discsare the same, two discs rotating in the same direction give rise to apetal scan pattern, as shown in FIG. 2a. If the angles of inclination ofthe two discs are the same and the discs rotate at the same speed but inopposite directions an almost linear scan pattern may be achieved, asshown in FIG. 2b.

For operation at millimetre wavelengths the apparatus shown in FIG. 1 isrequired to be large and, furthermore, is rather complex. It istherefore impractical for use at these wavelengths. Referring to FIG. 3,a compact scanning apparatus, suitable for use in a millimetre waveimaging system, comprises two reflecting plates, for example discs 1 a,1 b, supported on a single axis 7 passing through the centre of thesurface of each disc 1 a, 1 b, a rotary mechanism 3 and a fixed, planemirror 8. Radiation 4 from the image scene falls onto the first rotatingmirror 1 a. Any one direction of the incident radiation 4 undergoes aconical scan on reflection and falls onto the plane mirror 8. From themirror 8 radiation is reflected to the second rotating disc 1 b where itis reflected to the collection optics 5 of the imager. From thecollection optics, radiation is focused to the receiver element 6 of theimaging system situated in the image plane of the focusing optics 5.

The normals, n_(a),n_(b), to the two discs 1 a, 1 b make an angleθ_(a),θ_(b) respectively to the axis of rotation 7. For theconfiguration shown in FIG. 3, where the discs 1 a, 1 b are inclined inopposite directions, the direction of the scan is perpendicular to theplane containing both the axis of rotation 7 and a normal to the planeof the mirror 8. If the two mirrors are tilted in the same direction,rather than in opposite directions, the direction of the scan is in theplane containing both the axis of rotation 7 and a normal to the planeof the mirror 8. Typically, the angles θ_(a), θ_(b) may be between 1°and 10°.

It is advantageous to have the angles of inclination (θ_(a), θ_(b)) ofthe rotating discs 1 a, 1 b in the same plane and of substantially thesame amount (θ_(a)=θ_(b)=θ) but in opposite directions. In this case,the forces due to the tilt of the mirrors and their windage cancel onthe axis of rotation 7. With the configuration shown in FIG. 3, theincident beam of radiation 4 is scanned through an angle of ±4θ(whereθis the angle of inclination of the mirrors to the normal to the axis ofrotation 7). Therefore, for example, an inclination of 4° produces atotal field of view of 32° in the scene.

After radiation 4′ has been reflected from the two reflecting discs 1a,1 b, it is focused by the collection optics 5 onto the receiver 6 ofthe imaging system. The receiver 6 may typically be one or moremillimetre wave detectors in an array. A temporal encoded form of theimage is recorded by the detector or detectors in the image plane and,from a knowledge of the scan pattern, a two dimensional image may beunfolded from the temporal encoded signal or signals.

For particular disc angular velocities and phases the resulting scanpattern is a raster scan. For reasons associated with the way the eyeprocesses a moving image, a raster scan may be the most desirable formof scan. Furthermore, using a raster scan a linear array of detectorscould be used, each detector recording one or several lines in theimage. This architecture eases the unfolding of the data to form therequired image.

For example, with the two discs (FIG. 3) rotating at the same speed, theresulting scan is a line scan in one dimension. The second dimension inthe image may be formed by a linear array of detectors positioned at 90°to the line scan. In this case the number of image pixels in onedirection would be the same as the number of detectors.

In an alternative embodiment of the scanning apparatus, the two rotatingdiscs may be replaced with just one rotating disc 1, as shown in FIG. 4,further reducing the size of the entire apparatus. In thisconfiguration, the apparatus also comprises a 90° (ρ/2) roof reflector9.

The construction of the roof reflector 9 is described with reference toFIG. 5. The roof reflector 9 may comprise two flat reflective surfaces10 a, 10 b which are inclined at substantially 90° to each other and arein contact along an apex 11. In the figure, a hypothetical line 12 isdrawn between the two reflective surfaces 10 a, 10 b, wherein the line12 is substantially orthogonal to the apex 11. The line 12 shallhereinafter be referred to as the line of intersection of the twosurfaces 10 a, 10 b.

Referring to FIG. 4, radiation 4 from the scene is incident on the disc1 and is reflected to the 90° (ρ/2) roof reflector 9 where it reflectedback to the rotating disc 1 and then reflected to the collection optics5 of the imaging system, via a beam splitter 13 which separates the pathof incoming and outgoing radiation. Although it is preferable toseparate the path of input radiation 4 from the path of output radiation4′, in some operating configurations it may not be essential and thebeam splitter 13 may be omitted from the apparatus shown in the FIG. 4.

As in the previous example, the rotating disc is inclined slightly tothe normal to the axis of rotation 6 by an angle θ. Typically, the angleof inclination, θ, may be 5°. Using this configuration, an almost linearangular scan (as shown in FIG. 2(b)) is achieved in a plane parallel tothe line of intersection 12 of the two reflective surfaces 10 a, 10 b.

Although it is preferable to use a roof reflector in this arrangement,two independent reflective surfaces may also be used, where the tworeflective surfaces are inclined such that they are at an angle ofsubstantially 90° to each other but are not necessarily in contact. Thisarrangement, however, would result in a loss of some radiation reflectedfrom the disc to the reflective surfaces.

The beam splitter 13 may be a conventional polarising mirror andprovides a means of separating output radiation 4′, for transmission tothe imaging system, from input radiation 4. A conventional polarisingmirror typically consists of a flat transparent plastic sheet withclosely spaced, thin, parallel conducting wires. If the wires areoriented at an angle of 45° (ρ/4) to incident radiation, only 45° linearpolarised radiation is transmitted. The parallel conducting wires of thepolarising mirror are oriented at an angle of 45° to the incidentradiation 4, and therefore only 45° linear polarised radiation thereforepropagates to the roof reflector 9. Transmitted radiation is thereforeincident at the roof reflector 9 with its polarisation inclined at 45°to the line of intersection 12 of the two reflective surfaces 10 a, 10b. Radiation 4 experiences a 90° rotation of its direction ofpolarisation on reflection at the roof reflector 9 and is transmitted tothe reflective disc.

Upon reflection from the rotating disc 1 for this second time radiationis therefore ρ/4 linearly polarised and is subsequently reflected by thepolarising mirror 13 and passed to the collection optics 5. Thepolarising mirror 13 is therefore transparent for incoming radiationpolarised in a direction perpendicular to the direction of theconducting wires and is reflective for incoming radiation polarisedparallel to the direction of the conducting wires. The arrangement shownin FIG. 4 would therefore only allow a single polarisation to bedetected at the receiver 6.

In this embodiment, the rotating disc 1 has to be over-dimensionedcompared to the aperture of the collection optics 5, firstly because itsaxis of rotation is inclined to the direction of the incident andreflected beams and secondly because there is a significant displacementof the beam from its mean position as the mirror rotates about its axis.

Both of these effects may be overcome by placing the rotating disc 1close to the roof reflector 9, as shown in FIG. 6. As in the previousexamples, the normal to the rotating disc 1 is slightly inclined at anangle, θ, to the axis of rotation 7. In this configuration, the scanningapparatus includes a polarising roof reflector 14 comprising twosubstantially flat polarisers 15 a, 15 b inclined at substantially 90°to each other. The two polarisers 15 a, 15 b take the place of the tworeflective surfaces 10 a,10 b in FIG. 5. The polarisers 15 a,15 b havepolarisation axes oriented to transmit radiation having substantiallythe same polarisation and substantially parallel or perpendicular to theline of intersection 12 of the two polarisers 15 a, 15 b, thereforesubstantially perpendicular or parallel to the apex 11 (see FIG. 5). Asmentioned previously, it would also be possible to use two independentpolarisers in place of the polarising roof reflector, where the twopolarisers are inclined at substantially 90° but are not necessarily incontact.

In this embodiment, the scanning apparatus also comprises a Faradayrotator 16 for rotating the direction of polarisation of radiation by45° (ρ/4). Radiation incident on the Faraday rotator 16 undergoes arotation in its direction of polarisation each time it passes through(i.e. 45° rotation per pass). Radiation 4 having one particulardirection of polarisation is input through the roof reflector 14 to theFaraday rotator 16. Radiation is reflected by the rotating disc 1 andits direction of polarisation is therefore rotated by a further 45° asit is transmitted back through the Faraday rotator 16. The radiation isthen reflected at the roof reflector 14 and experiences a further totalrotation in its direction of polarisation of 90° as it passes back andforth through the Faraday rotator 16, while being reflected for a secondtime at the rotating disc 1. At this point, the direction ofpolarisation is such that radiation 4′ is able to pass through the roofreflector 14.

Alternatively, the Faraday rotator 16 may be replaced with a millimetrewave birefringent surface, such as a Meander-line. For incident planepolarised radiation, which may be resolved into two perpendicularcomponents each oriented at ρ/4 (45°) to the direction of polarisationof the incident beam, a meander line may be constructed to introduce a90° (ρ/2) phase shift between the two perpendicular components. A 90°(ρ/2) phase shift is therefore introduced in the state of polarisationof radiation each time radiation passes through the Meander-line.Further details relating to Meander-lines may be found in the followingreferences; L. Young et al., IEEE Transactions on Antennas andPropagation, vol AP-21, pp 376-378, May 1973, and R-S Chu et al., IEEETransactions on Antennas and Propagation, vol AP-35, No 6, pp 652-661,June 1987.

Having passed through the roof reflector 14, plane polarised radiationincident on the Meanderline is therefore circularly polarised. Thecircularly polarised radiation is reflected from the rotating disc 1 andpasses back through the meander line to the polarising roof reflector 14where it is reflected on the first pass, back through the Meander-lineand the reflective disc, but is transmitted on the subsequent pass.

In practice, a number of Meander-lines may need to be used in a stackedconfiguration to give the required ρ/2 phase shift between the two axes.The Meander-lines may be more suitable for use in millimetre waveimaging at the long wavelength end of the wave band (e.g. 35 GHz).

The path of output radiation 4′ reflected from the scanning apparatus isseparated from the input radiation 4 using an inclined flat polariser 17and an additional 45° Faraday rotator 18. Output radiation 4′ istherefore separated from the path of input radiation 4 and is directedto the collection optics 5 of the imaging system. In this configuration,it is essential that the polariser 17 reflects radiation atsubstantially 45° to the direction of polarisation transmitted by thetwo polarisers 15 a,15 b (i.e. at 45° to the apex 11). When using thisroof reflector 14 the direction of the scan at the imager is parallel tothe line of intersection 12 of the two polarisers 14 a, 14 b of the roofreflector 15. In this configuration, the imaging system will detect asingle polarisation state only.

Although it is preferable to separate the path of input radiation 4 fromthe path of output radiation 4′, in some operating configurations it maynot be essential and the polariser 17 and the Faraday rotator 18 maytherefore be omitted from the apparatus shown in the FIG. 6.

FIG. 8 is a modification of FIG. 6 and includes powered opticalcomponents to enable focused radiation to be passed directly to thereceiver 6. For example, FIG. 7 shows a reflector lens 19 which may beincluded in the scanning apparatus. The reflector lens 19 comprisesthree elements; two polarising elements 20,22 (alternatively referred toas polarising reflectors) and a Faraday rotator 21 which rotates theplane of polarisation of radiation passing through by 45°. The arrows23,24 indicate the direction of polarisation of radiation transmitted bythe elements 20 and 22 respectively.

For the purpose of this description, the elements 20,21,22 may also bereferred to as surfaces 20,21,22. Although the surfaces 20,21,22 areillustrated in FIG. 7 as having curved surfaces, this is not essential.For example, at least one of the surfaces 20,21,22 may have asubstantially planar surface.

The arrows shown along the path of radiation 4 indicate the direction ofpolarisation as the radiation is transmitted through the reflector lens19. Radiation 4 is incident on the first element 20 where one directionof polarisation is transmitted (i.e. radiation having a its direction ofpolarisation vertically in the plane of the paper). Radiationtransmitted by the first element 20 passes through the second element 21which rotates the direction of polarisation by 45°. For example, thesecond element may be a 45° Faraday rotator. The polarisation ofradiation incident at the third element 22 is perpendicular to thepolarisation state which is transmitted by the surface 20 and istherefore reflected. On the return path, radiation undergoes a furtherrotation of 45° in its direction of polarisation as it passes throughthe second element 21. The direction of polarisation is nowperpendicular to the transmission axis of the first element 20 and sothe radiation is reflected. The reflected beam undergoes a furtherrotation of 45° as it passes through the second element 21 and itspolarisation is such it is then transmitted, and output from thereflector lens 19, by the third element 22. Hence, the operation of thelens arrangement 19 is such that one polarisation passes through thelens without any focussing effect but when the same polarisation passesthrough a second time, on the return path, it is focussed. Thenon-recipirocal nature of the lens is achieved by using a Faradayrotator inside the arrangement.

FIG. 8 shows the still more compact scanning apparatus, including thereflector lens 19 shown in FIG. 7. The reflector lens 19 is situateddirectly in front of the roof reflector 14. If the surfaces of 20,21,22are of appropriate shape, radiation transmitted through the reflectorlens 19 will be focused. Incoming radiation 4, having the correctdirection of polarisation, is transmitted through the reflector lens 19and suffers no deviation while outgoing radiation 4′ is focused directlyto the receiver 6.

When the polarising roof reflector 14 is employed, the beam of radiationincident on the rotating disc 1 undergoes a considerable displacementalong the length of the rotating disc 1. Referring to FIG. 9, it ispossible to replace the single roof reflector 14 with a series of roofreflectors 25 of smaller dimension so that upon reflection from therotating disc 1 radiation is displaced by a reduced amount (the path ofradiation is not shown for clarity), therefore reducing the size of thescanning apparatus still further. Again, the reflector lens 19 may beused to focus outgoing radiation 4′ directly to the receiver 6.

The rotating disc 1 in FIG. 9 may be slightly concave. In this case, itis possible to achieve the near linear open scan pattern shown in FIG.10. This open scan pattern enables the number of television linesobtained with the scan pattern in FIG. 2(b) to be doubled. For example,for a detector array comprising a number of detector elements separatedby a pitch distance, d, matching the width, w, of the open scan patternto half of the detector pitch, d, enables an interlaced pattern to beobtained. Hence the maximum spatial frequency performance may beachieved. This is analogous to the microscan technique used in infraredimaging [D.J Bradley and P.N. J Denis, “Sampling effects in HgCdTe focalplane arrays in IR technology and applications” (Ed. L.R. Baker and A.Mason), Proc. SPIE vol 590 pp 53-60 (1985)].

The use of multiple roof reflectors in the arrangement of FIG. 9 canintroduce phase changes which impair the spatial resolution of theimager. It may therefore be preferable to sacrifice the benefit of thereduced size of the apparatus in FIG. 8 and to use only a single roofreflector, as shown in FIG. 8. However, the configuration shown in FIG.8 can lead to pupil wander due to the displacement of an incoming beam 4by the disc 1 and the roof reflector 15 a, 15 b arrangement andtherefore the effective pupil area of the system is reduced.

The apparatus may also be configured to provide a conical scanningsystem, rather than a raster scan. One configuration for achieving thisis shown in FIG. 11. This arrangement provides advantages over theapparatus shown in FIG. 8 in that it is more compact and does not giverise to pupil wander. It also has a much improved spatial resolutionover the apparatus. of FIG. 9.

The arrangement shown in FIG. 11 comprises a detector array 30 having anumber of detector elements 31, a reflector lens 19, and a rotatingplate or disc 1. The disc 1 typically rotates about an axis passingthrough its centre at an angle of inclination of a few degrees to thenormal to the axis, say 5°, as described previously. The reflector lens19 has the structure described with reference to FIG. 7 and comprises apolarising reflector element 20 (e.g. a vertical wire grid), a 45°Faraday rotator 21 and a polarising reflector element 22 (e.g. a 45°wire grid). The elements may have curved surfaces as shown in FIG. 7.Alternatively, one or two of the elements may have a plane surface.

The operation of the reflector lens 19 is such that incident radiation 4of one polarisation, in this case horizontal polarisation, passesthrough the lens arrangement without any focussing effect, as describedpreviously, whereas on passing through the lens for a second time, fromthe opposite direction, it is focussed.

A single detector element 32 in the detector array 30 traces out acircular scan pattern. As the detector elements 31 lie adjacent to oneanother, the image formed is a series of displaced circles, as shown inFIG. 12. As the reflector lens 19 can be placed between the detectorarray 30 and the rotatable disc 1 the scanning system is compact. Inconventional arrangements, scanning optics have to be located apart fromthe focussing components which can make such systems inconvenientlylarge.

In a particular embodiment of the arrangement shown in FIG. 11, thepolarising element 20 may have a substantially flat surface and thepolarising element 22 may have a substantially spherical surface, thisspherical surface having a radius of curvature R and thus a focus at adistance R/2 from the spherical surface. In this embodiment, theelements 31 of the detector array 30 form part of a spherical surfacehaving half the radius of curvature (R/2) of polarising element 22 andbeing concentric with it. The detector elements 31 may be fed by hornsin which case the apparatus is arranged such that the focus (i.e. atR/2) of polarising element 22 is located within the dimension of thehorn. As a further refinement, a corrector plate may be placed betweenthe rotatable disc 1 and the polarising element 22 to remove sphericalaberrations from the image formed at the detector array 30.

In any of the arrangements shown in FIGS. 8, 9 or 11 two or morereflector lenses 19 may be included in series.

For some applications the conical scanning apparatus shown in FIG. 11may be preferred over the FIG. 8 and 9 configurations, even at theexpense of the more complex conical scan pattern. In practice, thepreferred configuration of the apparatus will depend on the particularapplication for which it is to be used.

Whilst the scanning apparatus has been described with reference tomillimetre wave imaging in particular, it may also be applicable toother radiometry systems. The technique of transmitted high poweredradio waves to a scene and analysing radiation transmitted back to aradar receiver is well known. For example, by scanning radiationtransmitted back to the radar receiver using the scanning apparatus, theneed for large, moveable receiver elements employed in radar systems isremoved. The input radiation to the scanning apparatus is therefore theradiation reflected from the scene which is transmitted to the scene bythe radar transmitter. For the purpose of this specification the phrase“radiation from a scene” shall therefore be taken to mean radiationemitted by, reflected from or transmitted from a scene.

What is claimed is:
 1. An apparatus for scanning radiation from a sceneand for generating output radiation for input to a receiver systemcomprising; a first rotatable reflective plate, for receiving andreflecting radiation, having an axis of rotation passing substantiallythrough the centre of the plate, wherein the axis of rotation isinclined at a non-zero angle θa to the normal to the reflective plate,rotary means for rotating the reflective plate, secondary reflectionmeans for receiving and reflecting radiation and static reflection meansfor receiving radiation reflected from the first rotatable reflectiveplate and reflecting radiation towards the secondary reflection means,characterised in that the secondary reflection means is a secondrotatable reflective plate having a common axis of rotation with thefirst rotatable reflective plate, wherein the common axis of rotation isinclined at a non-zero angle θb to the normal to the second reflectiveplate.
 2. The apparatus of claim 1, and also including a millimetrewavelength imaging camera.
 3. The apparatus of claim 1, and alsoincluding a radar receiver.
 4. The apparatus of claim 1, wherein thenormals to the first and second reflective plates are inclined insubstantially the same plane and at substantially equal angles to thecommon axis of rotation and in substantially opposite directions.
 5. Theapparatus of claim 4 wherein the angles of inclination θa, θb arebetween 1° and 10°.
 6. The apparatus of claim 4 wherein the staticreflection means comprise a plane mirror having a reflective surfacesubstantially parallel to the common axis of rotation.
 7. The apparatusof claim 1, wherein the secondary reflection means is the firstrotatable reflective plate.
 8. The apparatus of claim 7 wherein the axisof rotation is inclined at an angle of between 1° and 10° to the normalto the reflective plate.
 9. The apparatus of claim 7 wherein the staticreflection means include two reflective surfaces inclined atsubstantially 90° to each other.
 10. The apparatus of claim 9 whereinthe two reflective surfaces form a roof reflector and are in contactalong an apex.
 11. The apparatus of claim 9 and further comprising apolarising mirror arranged to reflect output radiation to the receiversystem.
 12. The apparatus of claim 11 wherein the polarising mirror is asheet of plastic material comprising a plurality of parallel conductingwires, wherein the parallel conducting wires are oriented atsubstantially 45° to the apex of the roof reflector.
 13. The apparatusof claim 7 wherein the static reflection means include two polarisers,each having a polarisation axis, wherein said polarisers are inclined atsubstantially 90° to each other.
 14. The apparatus of claim 13 whereintwo polarisers form a polarising roof reflector and are in contact alongan apex, wherein the polarisation axes of the polarisers are oriented totransmit radiation having substantially the same direction ofpolarisation wherein the direction of polarisation is substantiallyparallel or substantially perpendicular to the apex.
 15. The apparatusof claim 7 wherein the static reflection means comprise a plurality ofpolarising roof reflectors, each comprising two polarisers and eachpolariser having a polarisation axis, wherein the polarisers areinclined at substantially 90° to each other and are in contact along anapex, wherein the polarisation axes of the polarisers forming each roofreflector are oriented to transmit radiation having substantially thesame direction of polarisation wherein said direction of polarisation issubstantially parallel or substantially perpendicular to the apexes. 16.The apparatus of claim 14, and further comprising a first Faradayrotator, situated between the polarising roof reflector and therotatable reflective plate, for rotating the direction of polarisationof radiation through substantially 45° each time the radiation passesthrough the Faraday rotator, such that the radiation having a particulardirection of polarisation may be output through the polarising roofreflector.
 17. The apparatus of claim 14, and further comprising; one ormore birefringent surfaces situated between the polarising roofreflector and the rotatable reflective plate, for receiving radiation ina state of polarisation, P_(s,) whereby the one or more birefringentsurfaces introduce a substantially 90° phase shift in the state ofpolarisation, P_(s) each time radiation passes through the one or morebirefringent surfaces, such that radiation having a particular directionof polarisation may be output through the polarising roof reflector. 18.The apparatus of claim 17, wherein the one or more birefringent surfacesare Meander-lines.
 19. The apparatus of claim 15, and further comprisingmeans for selectively transmitting radiation input to the apparatushaving a particular direction of polarisation and for selectivelyreflecting radiation output from the apparatus having a particulardirection of polarisation.
 20. The apparatus of claim 19 comprising; asecond Faraday rotator for rotating the direction of polarisation ofradiation output from the polarising roof reflector throughsubstantially 45° and also comprising a second polariser, wherein thesecond polariser has an axis of polarisation inclined at substantially45° to the apex or apexes of the one or more polarising roof reflectors.21. The apparatus of claim 7, comprising a lens arrangement forselectively transmitting and focusing radiation having a particulardirection of polarisation.
 22. The apparatus of claim 21 wherein thelens arrangement is a reflector lens comprising; a first polarisingsurface having a polarisation axis, for selectively transmitting andselectively reflecting radiation having a particular direction ofpolarisation, a second surface for rotating the direction ofpolarisation of radiation through substantially 45° and a thirdpolarising surface for selectively reflecting and selectivelytransmitting radiation, wherein the third polarisation axis makes anangle of substantially 45° with the first polarisation axis.
 23. Theapparatus of claim 21 comprising two or more lens arrangements arrangedin series.
 24. The apparatus of claim 7, wherein the static reflectionmeans form part of the lens arrangement, the apparatus being arranged toprovide a conical scanning apparatus.
 25. The apparatus of claim 24,wherein the first polarising surface has a substantially flat surfaceand the third polarising surface has a substantially spherical surfacehaving a radius of curvature, R, and also comprising a detector arrayforming part of a spherical surface having half the radius of curvatureof the spherical surface of the third polarising surface and beingconcentric with it.
 26. The apparatus of claim 25, and furthercomprising a corrector plate located between the rotatable disc and thethird polarising surface for removing spherical aberrations from animage formed at the detector array.
 27. A reflector lens comprising; afirst polarising surface having a polarisation axis, for selectivelytransmitting and selectively reflecting radiation having a particulardirection of polarisation, a second surface for rotating the directionof polarisation of radiation through substantially 45° and a thirdpolarising surface for selectively reflecting and selectivelytransmitting radiation, wherein the third polarisation axis makes anangle of substantially 45° with the first polarisation axis.
 28. Thereflector lens of claim 27, wherein at least one of the first, second orthird surfaces has a curved surface.